This invention relates generally to nuclear reactors and more particularly, to guide pins located between the shroud head flanges and the top guide in a boiling water reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The core center axis is substantially coaxial with the center axis of the shroud, and the shroud is open at both ends so that water can flow up through the lower end of the shroud and out through the upper end of the shroud. The shroud, top guide, and core plate limit lateral movement of the core fuel bundles.
The upper portion of the shroud, commonly referred to as the shroud head, is coupled to the top guide. Particularly, the shroud head flange mates with the top guide flange. The top guide flange typically includes cylindrically shaped guide/shear pins, and the shroud head flange includes corresponding cylindrically shaped mating holes. The purpose of the guide pins is to align the shroud head in the proper position during assembly of the reactor. Additionally the pins provide support and rigidity to the structure during a seismic event. Typically small clearances between the pins and their corresponding mating holes minimize the impact loading on the pins and flange holes caused by horizontal seismic accelerations. The small clearances make routine refueling assembly and disassembly operations difficult.
During assembly, the shroud head is typically suspended from an overhead crane and lowered into mating position with the top guide flange. Because of the large size and weight of the shroud head, the shroud head flange may flex between the lifting points. The flexing of the shroud head flange may cause the cylindrical holes to become misaligned with the guide pins on the top guide flange. The misalignment causes an interference condition which may not allow the top guide flange and the shroud head flange to mate properly.
It would be desirable to provide a shroud head and a top guide that include corresponding mating guide pins and flange holes that have a suitable clearance to accommodate any flexing of the shroud head flange during assembly. It would also be desirable to provide a shroud head and a top guide that include corresponding mating guide pins and flange holes that have small clearances after assembly to minimize the impact loading on the alignment pins and flange holes caused by horizontal seismic accelerations during a seismic event.